The disclosures herein relate generally to free span buildings, that is, buildings with no intermediate vertical supports, and more particularly to the connector hubs and steel tubes used in the space frame construction of such buildings.
In space frame construction, a generally cylindrical connector hub includes a plurality of outwardly directed slots extending along the peripheral surface of the connector hub. The slots have opposed ribbed surfaces. Tubular frame members are flattened and crimped at their opposed ends. The crimped ends include elongated flat surfaces extending outwardly, or away from each other. The crimped ends are ribbed in a pattern which can be mated into engagement with the ribs in the connector hub slots. In this manner, each end of a tubular frame member may be slidably inserted into a respective connector hub slot, and several tubular frame members may be connected at one end to a connector hub slot to form a spider, i.e., a connector hub having a plurality of tubes extending outwardly therefrom, each tube terminating at a free end.
The free end of each tube can be similarly connected to another connector hub. Thus, a framework of interconnected spiders formed of tubes and connector hubs can be joined to form a pre-assembled or modular section of a flat roof, a domed roof, a wall, etc., to be joined with other sections to eventually form a complete structure. The structure once completed is then covered with a selected cladding which is attached to the structural framework by means of an interfacing cladding support system.
The cladding may be fabric, corrugated metal sheets, glass, or other selected materials, and may include combinations of these materials for architectural design purposes.
It is important that each end of a tubular frame member be slidably inserted by hand into a respective connector hub slot, so that an entire building may be constructed without special equipment, and with unskilled labor. Prior art designs of connector hub slots do not account for the "flash" (a small fragment of metal) which the manufacturing process leaves at the flattened, crimped ends of the tubular frame members. Thus, often the persons assembling the space frame construction are unable to insert the end of a tubular frame member into a connector hub slot without using hammers and excessive force, or without first grinding off the extra fragment of metal at the ends of each tubular frame member.
Another problem with prior art designs is that load stresses cause the connector slot to open up and loose engagement of one or more teeth before the load reaches substantially the yield strength of the tube, resulting in non-ductile failure and reduced strength.
Another problem with prior art designs is that no account is made of differing material properties that may exist between the tubular member material and the connector material, again resulting in failure of one of these before the tube reaches substantially the yield strength of the tube, resulting in non-ductile failure and reduced strength.
What is needed is a connector hub slot that can accommodate the flash, and which will not release a tubular frame member before the load stresses reach at least 100% of the tubular frame member's load capacity as measured by its rated minimum yield strength times its area. Also, what is needed is an elongated tube end and connector slot combination that will produce ductile failure under tensile stresses.